For me, the finishing blow was a comment from an Opera
employee, in response to my complaints about the regressions in standard
support when Opera switched from Presto to Blink as rendering engine:

You may have lost a handful of things, but on the other hand you have
gained a lot of other things that are not in Presto. The things you
have gained are likely more useful in more situations as well.

This was shotly after followed by:

Whether you want to use Opera or not is entirely up to you. I merely
pointed out that for the lost standards support, you are gaining a lot
of other things (and those things are likely to be more useful in most
cases).

Other things? What other things?

But it gets even better. I'm obviously not the only one complainig about
the direction the new Opera has taken. One Patata Johnson
comments:

There used to be a time when Opera fought for open standards and
against Microsoft's monopol with it's IE. Am I the only one who us
concerned about their new path? Today Google / Chrome became the new
IE, using own standards and not carrying about open web standards that
much.

The reply?

Opera is in a much better position to promote open standards with
Blink than with Presto. It's kind of hard to influence the world when
the engine is basically being ignored.

Really? How does being another skin over Blink help promote open
standards? It helps promote Blink experimental features, lack of
standard compliance, and buggy implementation of the standards it does
support. That does as much to promote open standards as the Trident
skins did during the 90s browser wars.

As small as Opera's market share was before the switch, its rendering
engine was independent and precisely because of that it could be used to
push the others into actually fixing their bugs and supporting the
standards. It might have been ignored by the run-of-the-mill web
developers, but it was actually useful in promoting standard
compliance by being a benchmark against which other rendering engines
were compared. The first thing that gets asked when someone reports a
rendering issue is: how does it behave in the other rendering engines?
If there are no other rendering engines, bugs in the dominant one become
the de facto standard, against the open standard of the specification.

With the switch to Blink, Opera has even lost that role. As minor a
voice as it might have been, it has now gone completely silent.

And let's be serious: the rendering engine it uses might not be ignored
now (it's not their own, anyway), but I doubt that Opera has actually
gained anything in terms of user base, and thus weight. If anything, I'm
seeing quite a few former supporters switching away. Honestly, I suspect
Opera's survival is much more in danger now than it was before the
switch.

The truth is, the new Opera stands for nothing that the old Opera
stood for: the old Opera stood for open standards, compliance, and a
feature-rich, highly-customizable Internet suite. The new one is
anything but that.

At the very least, for people that miss the other qualities that made
Opera worthwile (among which the complete, highly customizable user
interface, and the quite complete Internet suite capabilities, including
mail, news, RSS, IRC and BitTorrent suport) there's now the open-source
Otter browser coming along. It's still WebKit-based), so it
won't really help break the development of a web monoculture, but it
will at least offer a more reliable fallback to those loving the old
Opera and looking for an alterntive to switch to from the new one.

For my part, I will keep using the latest available Presto version of
Opera for as long as possible. In the mean time, Firefox has shown to
have most complete support for current open standards, so it's likely to
become my next browser of choice. I will miss Opera's UI, but maybe
Otter will also support Gecko as rendering engine, and I might be able
to get the best of both world.